Signal-responsive apparatus having magnetic-particle-friction memory

ABSTRACT

Signal-processing apparatus for use with industrial instrumentation systems and the like, and comprising memory means to maintain a signal level for long time periods without drift. An input signal level is translated into the positioning of a movable member, through the use of an actuator motor controlled by the input signal and a position feedback signal. A friction device is coupled to the movable member to hold it in any given position, thereby to serve as a memory for the input signal level. The friction device comprises a thin vane-like element which is secured to the movable member and disposed in the air-gap of a permanent magnet. The air-gap also includes a mass of tiny magnetizable particles which, under the influence of the magnetic field, engage the surfaces of the vane-like element to develop a frictional restraining force. This frictional restraint has the characteristic of being relatively low upon initial movement away from a start point, and thereafter increasing progressively to a plateau level, thereby providing superior dynamic performance particularly in a servo positioning system. Various applications are disclosed including process controllers and chart recorder systems.

This is a continuation, of application Ser. No. 543,443 Filed Jan. 23,1975, now abandoned, which in turn is a continuation application Ser.No. 333,997 filed on Feb. 20, 1973, and now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to signal-handling apparatus used, for example,in developing and transmitting measurement and/or control signals ininstrumentation systems for industrial processes. More particularly,this invention relates to apparatus for translating, storing (as bymeans of a "memory" device), and reading-out signals the magnitudes ofwhich represent information of a numerical or quantitative nature, e.g.the magnitudes or other measurable status of physical conditions such aspressure, temperature, valve position, and the like.

In many instrumentation systems, it is necessary to develop and toprocess (i.e. to alter in successive steps or stages) signalsrepresenting quantitative information such as the magnitudes,intensities, etc., of specific physical conditions. The requiredprocessing or handling of such signals typically involves various kindsof signal translations or transformations, wherein the character of thesignal is altered to provide certain desired attributes or to permitcertain computations or manipulations to be performed. In many cases, italso is necessary to incorporate as an integral element of thesignal-handling function the facility to "remember" the magnitude of asignal level for quite long periods of time, e.g. while associatedportions of a control system are devoted to different functions on atime shared basis, or, as a more extreme example, when there has been abreakdown in some part of the equipment, an electrical power failure, orother catastrophic event.

The required memory function in such systems generally will havestringent operational and performance specifications, most particularlywith respect to the ability to hold a signal level for long periods oftime without significant change for any cause, including drift effectswhich are associated with certain kinds of known memory elements. Inaddition, signal-handling apparatus of the type referred to herein musthave the capability of providing simple, accurate and non-destructiveread-out of the stored signal quantity, whereby the signal information(magnitude) is available in a format which is generally useful in (1)any further processing or signal handling, (2) the presentation of thesignal information visually, or (3) the transmission of the signalinformation to remotely-located associated equipment either on acontinuous basis or on an "as called for" basis.

2. Description of the Prior Art

As one example of a prior-art signal-processing apparatus having amemory function of the general class herein considered, reference may bemade to U.S. Pat. No. 3,550,014 (D. A. Richardson et al). In thatpatent, there is disclosed an electronic process controller of theanalog type wherein a condition measurement signal and a set-pointsignal are compared to produce a deviation signal which is processed byrate and reset means to develop a D.C. output signal directed to a flowvalve or the like, to position the valve in correspondence with themagnitude of the controller output signal. To accommodate and preparefor switch-over of the controller to manual operation, the controlleroutput signal also is directed to a so-called "memory capacitor"(reference number 74 in the abovementioned patent), to maintain thememory capacitor charged to a level corresponding to the valve controlsignal.

At switch-over of the controller to manual operation, the memorycapacitor referred to above, together with associated signal-producingcircuitry, takes over the function of furnishing the valve controlsignal, maintaining the control signal level at the magnitude it hadjust before switch-over to manual mode. In practice, the memorycapacitor and its associated circuitry are selected to be ofhigh-quality components, capable of holding the capacitor charge withonly moderate change over a relatively long period of time. Thus thecapacitor can for some time supply a nearly constant, continuous valvesignal, substantially equal to the controller valve signal at the momentof switch-over to manual operation.

However, the charge on any such memory capacitor ultimately willdissipate through leakage, even though slowly, so that the valve signalwill not remain exactly fixed in magnitude. The result is that themanual signal level must be readjusted from time-to-time, if thecontroller remains in manual mode for an extended period. Accordingly,there has been a need for a truly satisfactory drift-free memoryarrangement to hold the valve signal steady without introducing stillother complications or disadvantages. Any alternative memory arrangementparticularly should be simple in construction, inexpensive tomanufacture, and reliable in performance. Preferably, it should be ableto retain the valve signal information even in the face of power failureor other equipment malfunction.

As another example of a prior-art signal-handling apparatus having amemory device, reference may be made to U.S. Pat. No. 3,034,718 (M. P.Freitas et al). That patent describes a process control system of thetype which includes a centrally-located digital computer for determiningthe set points of a number of separate analog controllers. This computerproduces set-point adjustment signals which are transmittedperiodically, in sequence, to set-point stations associated withrespective analog controllers. These set-point stations provideset-point signals for the associated controllers.

Each set-point station also serves the function of a memory device, tostore the previously-established set point signal level for use by itsassociated controller during the times when no signal is received fromthe computer. The adjustment signals from the computer alter the storedsignal level whenever required, and the set point is held at the newadjusted level until the next adjustment signal is received. Theset-point signal level also can be adjusted manually, if desired.

The set-point station disclosed in the abovementioned Feitas et alpatent basically comprises a mechanical arrangement wherein aconstant-speed, reversible motor is responsive to a pulse-lengthcomputer signal, and serves to rotationally position a shaft carrying anoutput potentiometer. When the motor is not energized by the computersignal, it holds the shaft position fixed, thereby "storing" theset-point signal which is read-out by the potentiometer to theassociated analog controller. Thus, the reversible motor serves the dualfunction of rotating the output potentiometer, thereby altering theset-point signal, and also "remembering" the set-point signal by holdingthe shaft position fixed between adjustment signals from the computer.

SUMMARY OF THE INVENTION

Signal-processing apparatus in accordance with the present inventionwill include means responsive to some kind of condition for producing asignal indicative or representative of the magnitude or other measurableaspect of the condition. That is, the condition of interest is definablein terms of quantized, e.g. numerical, information, so that thecondition can be represented by a controllable (variable) signalcharacteristic which may be expressed in various ways, such as in termsof signal amplitude, frequency, or some other form of representationalvalue. The processing of this signal will include, as one stage or phasethereof, the storage of a corresponding signal level in a memory deviceas will be described herein, and which provides for essentiallydrift-free maintainence of the signal information for extended orunlimited periods of time.

Processing of the signal subsequent to storage in the drift-free memorydevice will include the function of converting and/or presenting thesignal information in a form adapted expressly for the intendedutilization, and thus the nature of such presentation will depend uponthe particular application. In some instances, the presentation of thestored information may be arranged as a visual signal suited for mentalcomprehension, e.g. the positioning of a pointer with respect to ascale. In other instances, the stored information will be transformed toa different signal format adapted for transmission to a remoteutilization device. Typically such signal will be in the form of acontrollable energy level or other suitable characterization.

It will be evident that in different parts of the signal-handlingapparatus, the active signals carrying the information of interest mayhave quite different forms. Thus they may be electrical in nature, theymay be pneumatic, they may be mechanical, they may be avisually-recognizable information signal, or they may be in still otherforms. The important factor is that such signals be adapted to conveythe desired intelligence relating to the input condition, that they besuited for any required processing, and that they be readilytranslatable into a form adapted for use with the drift-free memorydevice to be described.

The memory device forming part of one presently preferred embodiment ofthis invention comprises a movable member having an non-magneticblade-like vane element attached thereto and passing through a mass oftiny magnetizable particles in a magnetic field. The particles arethereby magnetized and form a friction-producing, gripping engagementwith the non-magnetic vane element, tending to hold that element, andthe movable member associated therewith, in a fixed position in theabsence of the input driving force. Such driving force may be developedin various ways, such as by an electrically-energizable, solenoid-likemotor which applies to the movable member a relativelyconstant-magnitude but directionally reversible force.

When the driving motor is energized in response to an input signal, themovable member is accelerated against the friction force of themagnetized particles until (if the force is of sufficient duration) themovable member reaches a velocity plateau, moving towards a new desiredposition. The position of the movable member is continuously sensed or"read out" by means such as a position-to-voltage converter (e.g. aHall-effect device), arranged to produce a read-out signal the magnitudeof which corresponds at all times to the position of the movable member.This voltage signal is, in certain servo-system embodiments to bedescribed, directed to the input of the motor control circuit as afeedback signal, to cause the motor force to drop to zero when thedesired position of the movable member has been reached. Such amagnetic-friction memory-arrangement provides significantly improvedservo performance, apparently particularly due to the uniquefriction-force characteristics provided by the magnetized particles, aswill be described in detail hereinbelow.

When there is no repositioning force applied by the driving motor, themagnetic-particle friction arrangement serves to hold the movable memberfirmly fixed in position. That is, the magnetic friction means performsa memory function, locking securely in a drift-free storage the signalinformation represented by the position of the movable member, as lastaltered by the signal-controlled driving motor.

The driving motor is controlled by an input electrical current derivedfrom a source arranged to develop a signal responsive to a physicalcondition such as a pressure, temperature, or the like. This signalsource can for example be a computer, or it can be any one of many kindsof components, such as an analog controller developing a valve controlsignal.

The memory apparatus described herein can be arranged to receive inputsin various formats, e.g. in the form of pneumatic signals or electricalsignals. The apparatus moreover can be so arranged that it can beoperated, in alternative modes, either (1) by a controllablecondition-responsive input signal, or (2) manually. For example, in asingle unit in accordance with the invention, the input force to themovable member can be derived from an electrically-controlled component,as a motor or the like, or, in a different operating mode of the sameapparatus, the movable member can be positioned by hand. In eitherevent, the magnetized particles and associated elements provide the samememory function.

The output from the movable member and associated magnetic frictionelements can be developed in a number of different formats. That is, theoutput can in typical applications be in the form of an electricalsignal (voltage or current), in the form of a light signal (e.g. acontrollable laser beam), in the form of a penumatic signal, or in otherenergy-representational forms. With suitable cooperating elements, theoutput signal can with advantage in certain applications be derived as avisual read-out signal, as in the positioning of a pointer with respectto a scale, or by controlling the positioning of the pen of a chartrecorder.

This magnetic memory means offers a number of significant advantagesbeyond its drift-free characteristic. Thus, the magnetic-particlefriction memory device will not wear, in the sense that ordinaryfriction devices wear after repeated usage. Also, operation of themagnetic memory is not, as a practical matter, affected by the intrusionof dirt particles, since the memory already consists of many particlesphysically similar to dirt particles. Moisture, including even oil (atleast in small quantities), does not adversely affect the performance ofthe magnetic memory means to be described. And the construction of themagnetic memory means does not require critical tolerances oradjustments.

Still other objects, aspects and advantages of the invention will inpart be pointed out in, and in part apparent from, the followingspecification considered together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing signal-handling apparatus in accordancewith the present invention;

FIG. 2 is a side elevation view of the apparatus of FIG. 1;

FIG. 3 is a detail elevation view taken along line 3--3 of FIG. 2 toshow aspects of the magnetic particle friction memory;

FIG. 4 is a detail elevation view taken along line 4--4 of FIG. 2 toshow the position read-out means;

FIG. 5 is a schematic block diagram illustrating further elements of thesignal-handling apparatus of FIG. 1;

FIG. 6 shows in schematic form details of a flow controller based on thepresent invention;

FIG. 7 shows schematically a computer-control system with analogcontroller back-up and manual control facilities all utilizing thepresent invention;

FIG. 8 is a block-diagram schematic illustrating a time-shared recordersystem utilizing the present invention;

FIG. 9 is a graph demonstrating certain force vs. distancecharacteristics of a magnetic-particle-friction memory device; and

FIGS. 10 and 11 are sketches to illustrate how the magnetic particlesare aligned and move in operation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 and 2, the apparatus shown there comprises anelongate member 20 pivotally mounted at 22, as by means of suitableflexures, for rotary motion in a horizontal plane. Such motion isimparted by an actuator which may take any of various forms, and isillustrated herein as an electrically-energized, moving-coil drivingmotor 24. The coil 26 of this motor is mounted on the member 20 andsurrounds a permanent magnet 28 fixed in position on the instrumentbase. Current flow through the motor coil develops a corresponding forceproportional to current magnitude, tending to rotate member 20 about itspivot axis in a direction which depends upon the direction (polarity) ofcurrent flow.

As discussed hereinabove, the positioning of the member 20 is in effecta signal representing a quantized magnitude or other item of informationrespecting some physical condition such as temperature or the like. Toinsure retention of this information under a variety of operatingconditions, the pivoted member 20 is coupled at its left-hand end to amemory device, generally indicated at 30. This device, althoughpermitting member 20 to be moved by motor 24, serves to hold member 20firmly in any given position to which it has been directed by thatmotor, or by any other source of input motion of appropriate strength.This holding action is produced by a frictional restraint which developssufficient restrictive force to maintain the movable member 20 securelyin any position, in the absence of a driving force from the motor orother input source of comparable strength.

The memory device 30 comprises, in this embodiment, a relatively thinblade-like vane element 32 of non-magnetic material (e.g. brass,stainless steel, etc.) which is secured to the member 20 for movementtherewith. Referring also to FIG. 3, this vane element 32 is disposedhorizontally within the air-gap of a permanent magnet 34, parallel tothe opposed pole-pieces 36, 38 of the magnet.

Also disposed within the air-gap of the magnet 34, and thus subject tothe approximately vertical magnetic field produced thereby, is a mass oftiny particles 40 of a magnetizable substance, substantially filling theair-gap spaces above and below the vane element 32. These particles mayfor example be of ferrite material having relatively small retentivity,and thus relatively small hysteresis. Very succesful results have beenachieved with commercially available iron-oxide particles of the typeused for making ferrite magnetic cores. One batch of particles used fortest purposes was specified as 60 mesh; examination of the particlesindicated that they were non-uniform in shape and size, some particlesdiffering perhaps as much as 50:1 in size. However, it is not presentlyconsidered that such non-uniformity is essential in carrying out thepresent invention.

The particles 40 are magnetized by the field produced by the permanentmagnet 34 and, under the influence of that field, they engage theadjacent surfaces of the vane element 32 with a frictional contact,thereby to develop a restraining force opposing (but not preventing) anymotion of the movable member 20. The amount of frictional restrainingforce is to some degree proportional to the intensity of the magneticfield which could, of course, be provided by an electro-magnet andthereby controllably altered to achieve any special effects forparticular applications. In any event, within a very wide range, ofmagnetic field strengths, the friction force developed by the particles40 serves positively to prevent any drift movement of member 20, whilepermitting desired movement responsive to an input force from thedriving motor. Thus, the magnetic-particle friction device servescontinuously as a "memory" to retain securely any signal magnitude orother information represented by the positioning of member 20 actingunder the impetus of the driving motor.

The information represented by the positioning of member 20 is read outby means operable to produce a signal having a controllablecharacteristic identifying that information. In the FIG. 1 embodiment,such read-out is effected in the first instance by a position-to-voltagesensor 50 which can be any of various known devices but isillustratively shown as a so-called Hall-effect generator. Such anarrangement comprises a crystal 52 of semi-conductor material mounted onan arm 54 integral with the movable member 20. As this member rotatesabout its pivot axis 22, the crystal 52 moves towards (or away from) theair-gap of a permanent magnet 56, to subject the crystal to a magneticfield intensity which varies as a function of the positioning of member20.

With a constant current flowing through the Hall-effect crystal 52 (fromconventional means, not shown in FIGS. 1 and 2), the application of suchmagnetic field from magnet 56 results in development of an outputvoltage at the crystal output terminals. The magnitude of this voltageat all times is proportional to the magnetic field intensity. Thus thevoltage magnitude at the sensor output corresponds at all times to theposition of the member 20.

The apparatus of FIGS. 1 and 2 includes an additional read-out means inthe form of a pointer 58 on the right-hand end of the pivoted member 20(see also FIG. 4). This pointer is positioned for horizontal movement infront of a conventional scale 60 illustratively arranged in this case toindicate valve positions between 0% and 100%. Thus the pointer-and-scalecombination provide a direct visual read-out of the position of member20, to indicate to an information.

The pointer 58 also can serve as a manually-controllable actuator meansfor the member 20. Thus an operator can grasp the extended pointerportion of member 20 and move that member manually to any desiredposition. Wherever the pointer is so positioned manually, the magneticmemory means 30 will, with its continuous friction restraint, serve tohold member 20 securely in place, pending receipt of a further actuatorforce from the motor 24, or manually. It may further be noted that themember 20 can be provided with any other desired mechanical meansspecially designed to facilitate manual grasping of member 20 to effectany appropriate movement of member 20. In any event, regardless ofwhether the member 20 is moved by an electrical actuator, a pressure(e.g. pneumatic) actuator, a manual actuator, or any other form ofactuator, the sensor 50 will at all times produce an electrical outputsignal corresponding to the actual positioning of the movable member.

The voltage read-out signal from sensor 50 can be employed to performany of a variety of functions. One important function in manyapplications of the invention is to provide a position feedback signalfor use in servo control of the member 20. Such an arrangement isillustrated schematically in FIG. 5.

Referring first to the central portion of FIG. 5, there is shown withina dashed-line block 70 the basic elements previously described withreference to FIGS. 1 and 2. As shown in FIG. 5, the actuator motor 24receives its control current from an amplifier 72 the principal input towhich is derived through a resistor R₁ connected to a signal sourcegenerally indicated at 74. This signal source can be any of manydifferent kinds of equipment, such as a computer or an analog processcontroller, and as illustrated at 76 receives incoming data or signalinformation respecting the magnitude or other characteristic of aphysical condition concerning which computations or other signalprocessing must be performed.

The position of member 20 is at all times tracked by the sensor 50, andthe output signal of this sensor is directed to the input of anamplifier 78 to increase the signal energy level. The output of thisamplifier is shown connected to an output terminal 80 for subsequenttransmission to associated equipment for further processing orutilization. The amplifier output signal also is directed through aconductive negative feedback circuit 82 (including resistor R₂) to theinput of amplifier 72, so as to oppose and counter the input signalproduced by the signal source 74.

When the movable member 20 has been driven to a position producing afeedback signal from amplifier 78 equal to the input signal from source74, the amplifier 72 will no longer send current through motor coil 26,and the member 20 ultimately will come to rest in that commandedposition. The memory device 30 will thereafter automatically hold themember 20 securely in that position, preventing any drift or alterationof the information so represented. That information is, in turn,represented by the output signal from amplifier 78 and by the visualdisplay of pointer 58 with respect to scale 60.

An important attribute of the memory device 30, especially in a servosystem application such as shown in FIG. 5, is its unusual force vs.motion characteristic. Of particular importance is that motion of member20 can be initiated by a relatively low force, substantially less thanthat required to move the member large distances. To demonstrate thisaspect, FIG. 9 has been included to present a graph illustrating therelationship between force and movement of the vane 32, as it moves awayfrom, and returns to, an arbitrary "neutral" starting point.Specifically, this graph shows displacement of the vane plotted againstthe friction force opposing such movement (i.e. the force which isovercome by the motor 24, or manually, to produce the observedmovement).

As appears from this FIG. 9 graph, the amount of force required toinitiate movement is quite low, while the amount of force required tosustain movement progressively increases as the vane moves away from itsstarting or neutral position. Ultimately, as shown by the curveextension 90, the required force reaches a plateau where an appliedforce of that magnitude will maintain continuous motion. Any furtherincrease in force will accelerate the vane until some other limitingcondition is reached. If the force is reduced to zero, as at 92, thevane will stop. Movement in the reverse direction can be initiated by arelatively low force, while progressively higher friction force will beencountered as the vane moves still further in the reverse direction.

This characteristic is particularly well suited for servo applicationsbecause it tends to minimize hunting or other oscillations, and makes itmore readily possible to have a dead-beat (e.g. critically damped)positioning movement without any significant overshoot. In ordinaryservo systems, wherein conventional friction characteristics arepresent, the initial resisting friction force will be quite high(relative to the friction encountered at sustained velocities). Thus theservo motor must build up a high initial force to overcome the startingfriction. Subsequently, as the conventional friction force decreaseswith increasing velocity, the driven element tends to accelerate unduly,making de-hunting and stabilization difficult. These problems arelargely minimized or avoided by the magnetic-particle frictioncharacteristics shown in FIG. 9.

It should be understood that the graph of FIG. 9 has been provided tohelp explain certain observed advantages of the magnetic-particlefriction memory device 30, and that it presents a somewhat idealizedversion of the characteristics of that device. Thus, the graph has beendrawn to indicate a symmetry of response to movement in either directionwhich may not be entirely realizable in practice. Similarly, thehysteresis-like characteristics may not be exactly duplicated inrepeated cycles of the memory device 30.

In any event, there is some uncertainty at present regarding the precisetheoretical explanation for the characteristic behavior of the magneticmemory 30 as illustrated in FIG. 9. However, it would appear that atleast part of the explanation may be that the tiny magnetic particlestend, when in a magnetic field, to become aligned in chain-like stringsas indicated in FIG. 10. Evidently, such strings have a degree oflongitudinal extensibility so that, as suggested in FIG. 11, initialmovement of the vane 32 will be accompanied primarily be a lengthwiseextension of the strings, thus producing a relatively low oppositionforce against that initial movement. Thereafter, as more of the stringsprogressively reach the limit of their longitudinal extensibility, themagnetic particles engaged with the vane begin to slide over the vanesurface causing the more normal frictional characteristic which might beexpected from a mass of particles.

As noted hereinabove, the force produced by the driving motor 24 isproportional to the current flowing through coil 26. However, the gainof the amplifier 72 may typically be quite high, so that the motorcurrent will reach a limiting (maximum) value with a relatively smallamplifier input signal. Put in terms sometimes used in describing servosystems, the V notch of the motor operating characteristic ordinarilywill be very narrow, perhaps as small as 1/10% of full-scale. Thus, ifthe signal source 74 in such an arrangemnet commands a change inposition greater than 1/10% of full-scale movement, the motor currentwill quickly reach its maximum level. Since many changes in positioncould be expected to be larger than 1/10%, the driving motor will undersuch conditions appear to be essentially an on-off constant-forcedevice.

The magnetic memory 30 also has a characteristic "V" notch, in that thefriction force is minimum at the start position, and progressivelyincreases for movement in either direction away from that startposition. Apparently, the width of the V depends at least to some extenton the nature of the magnetic particles, particularly their size, andperhaps other aspects thereof. In one embodiment, the half-width of theV, i.e. the distance the vane moved from its start point to reach themaximum friction plateau, was about 0.01 inch to 0.02 inch. In thatparticular embodiment, such vane movement constituted about 2% to 4% offull-scale movement, substantially larger than the V of the drivingmotor characteristic. However, since the amount of movement required togo from minimum to maximum friction is in reality an absolute distance,the percentage of full-scale movement represented by that distance canbe altered by changing the total travel of the vane required forfull-scale output change.

When the V of the motor characteristic is significantly smaller thantthe V of the magnetic memory, the motor force will reach its maximumvalue well before the friction restraint reaches its maximum value. Toprevent excessive speed of the movable member 20 for large changes inposition, the maximum frictional force should be proportioned in somereasonable fashion with regard to the maximum motor force. For example,the maximum friction force may be set at about one-half the maximummotor force, thus providing some margin of additional motor force, e.g.to assure adequate acceleration, to overcome other frictions which willbe present in any practical installation, and to aid in overcoming thepossible effects of time lags such as due to the inductive reactance ofthe motor coil 26. It may also be noted that the back e.m.f. of themotor coil, due to motion of the coil in the permanent magnet field,tends to reduce the coil current in a dynamic state. This effect tendsto reduce the maximum available motor force at higher velocities and canprovide some degree of self regulation of the vane movement.

The system of FIG. 5 can be used as a process controller, to produce atoutput terminal 80 a valve control signal corresponding to a controlsignal developed by a conventional control-function-generator serving asthe signal source 74. Such control-function-generator may for example belike that shown in the above-mentioned U.S. Pat. No. 3,550,014,producing an output signal responsive to a measurement input signal,and, when appropriate, incorporating into the output signal suitablereset and rate components for the particular process application. Theapparatus 70 translates such signal into a directly correspondingposition information signal (i.e. the position of member 20), and thesensor 50 and the pointer 58 provide read-out signals representative ofthat position information signal. The read-out signal from amplifier 78can be directed to a remotely-located valve operator or the like toposition a process valve in accordance with the output signal magnitude.The pointer 58 can serve the function of indicating the position of thevalve, thereby dispensing with the usual valve position meter typicallyassociated with a process controller which is remote from the valve.

In a process control application of the FIG. 5 system as describedhereinabove, the magnetic-particle memory 30 serves at all times tostore a signal representing the valve control signal. Thus, in the eventof failure of power for the control-function-generator 74, the valveposition information is held securely in a non-destructable type ofmemory, for continuously generating a suitable valve control signal, orfor reconstituting such valve control signal after power has beenrestored.

The system can be placed in manual operation by opening a switch 94between the control-function-generator 74 and the input to amplifier 72,and thereafter controlling the valve signal by manually shifting thepointer 58 to any desired valve opening. In such manual mode, theoperator in effect grasps the process valve stem when he grasps thepointer 58, because movement of the pointer automatically causes acorresponding movement of the valve stem.

FIG. 6 shows a process controller arrangement wherein the feedback meansaround the memory apparatus 70 is non-conductive (specificallycapacitive), and plays a part in determining the dynamics of the controlfunction. The particular controller design shown can be used for variouscontrol applications, such as flow control, and thus incorporates meansto produce both proportioning and reset action in the valve controlsignal.

The deviation signal (i.e. a signal proportional to the differencebetween a measurement signal and a setpoint signal) is directed firstthrough an RC network consisting of a large resistor 100 (adjustable ifdesired) in parallel with a capacitor 102, and then through a relativelysmall input resistor 106 to an amplifier 108. This amplifier includes astabilizing feedback circuit 110, and a small capacitor connectedbetween its input terminals. The output of amplifier 108 controls thecurrent flow through an actuator motor coil 26 as before, to apply aforce to a movable member (not shown in FIG. 6, but comparable to member20 in FIG. 1). Associated with that movable member is a magneticparticle memory as at 30 in FIG. 1. Position sensor 50 is responsive tothe positioning of the movable member, and produces a signal which isdirected to an amplifier 112 to develop a valve control signal and acorresponding feedback signal.

In the FIG. 6 controller, the feedback signal from amplifier 112 iscoupled through a capacitor 114 to the junction between resistors 100and 106. (This junction is in effect the input of amplifier 108, becausethe input resistor 106 is of quite small size relative to resistor 100.)In operation, when there is a change in the deviation signal at thecontroller input, e.g. a step upset, this change in voltage initiallywill appear across input capacitor 102, and a correspondingopposite-polarity voltage will appear across feedback capacitor 114,i.e. at the output of the controller. The magnitude of the outputvoltage change will be determined by the relative capacities ofcapacitors 102 and 114. If they are equal, the controller output signalchange will be equal to the deviation signal change. (Note: This initialoutput signal change represents the "proportioning step" response to asudden change in measurement value.)

After the initial response described above, the feedback capacitor 114will charge up at a rate determined by the RC combination of resistor100 and capacitor 114. The feedback circuit around apparatus 70 servesin effect as an integrator, providing reset action in the controlleroutput to insure that the valve control signal ultimately reaches aproper magnitude to restore the measurement value substantially to thedesired set point level. Accordingly, it will be seen that apparatus inaccordance with the present invention is well suited for use in processcontrol systems.

The FIG. 6 system also includes means for transfer between automatic andmanual operation in a bumpless, balanceless fashion. By placing switch94A in manual position, the input of amplifier 108 is effectivelygrounded, and the motor coil 26 is de-energized. Thereafter, the movablemember 20 (as shown in FIG. 5) can be shifted manually to control theoutput signal to the valve. If desired, the principalcontrol-function-generator components (e.g. those to the left of line74A) can physically be removed to permit any repairs or adjustmentswhich might be necessary, all without disturbing in any way thecontinued operation of the output signal portions of the equipmentincluding the memory means 30 and the member 20. The controller canthereafter be placed back into operative condition and switched intoactive control of the driving motor 24 without "bumping" the process orrequiring any balancing adjustment prior to switchback to automaticoperation.

FIG. 7 shows the generic memory apparatus 70 used in a process controlsystem wherein the primary control signal is derived from a computer120, e.g. in the form of pulse-type signals, such as those wherein thepulse duration represents the amount by which the process valve is to beshifted, and the pulse polarity indicates the direction of valvemovement. The computer signals are directed to an input amplifier 122 tocontrol the operation of the movable member (e.g. member 20) theposition of which is maintained securely, between commanded positionchanges, by means of a magnetic particle memory 30. The position of themovable member is continuously sensed to produce an output valve controlsignal from an output amplifier 124.

A three-way switch 126 is provided to permit the system to be switchedfrom computer control to back-up analog control, or to manual control.In the latter position, a manually-operable two-position switch 128 canbe momentarily actuated to send current, of either polarity, through alarge resistor 130 to the input amplifier 122, to vary the position ofthe movable member of apparatus 70, and to alter the valve controlsignal correspondingly. Once the valve signal has been set to thedesired level, the memory apparatus 70 will maintain it securely at thatlevel, i.e. acting as a so-called "hard" manual. When switch 126 isplaced in back-up position, an analog controller is formed in anarrangement similar to that described with reference to FIG. 6. Sincethis controller normally would be used only in emergency conditions,when precision control is not likely to be essential, the variouscontroller components need not be of the most costly, highest qualitytypes. For example, the capacitors 132 and 134 could be ordinaryelectrolytic capacitors.

FIG. 8 illustrates how the memory apparatus of the present invention canbe applied to a chart recorder of the type wherein a pen producespermanent markings on a moving record. The system of FIG. 8 comprises aplurality of strip-chart recorder units 150A, 150B, 150C, each with itsown memory apparatus, and all serviced on a time-shared basis by commonoperating equipment symbolized by an amplifier 152.

Each recorder unit 150 includes a pivoted member 20 carrying a pen 154to produce traces on the moving chart record 156. The member 20, aspreviously described, is actuated by a driving motor 24, and itsposition is continuously tracked by a sensor 50 to produce a read-outsignal indicating the pen position. The movable member 20 also iscoupled to a memory device 30, utilizing the friction characteristics oftiny magnetized particles as previously described.

The time-sharing system may be any of many types used heretofore, andone form of system is shown in FIG. 8 simply to illustrate the concept.In this system, three multiplexing switches 160, 162, 164 (performing acommutator-like function) are controlled in unison by a switchsynchronizer 166 to: (1) connect each of the recorder units 150, insuccession, to the common operating equipment so as to supply a motorcontrol signal to the selected recording unit and to supply a penposition feedback signal from the selected unit to one input ofamplifier 152, and (2) connect the other input of amplifier 152 to ameasurement signal corresponding to the selected recorder unit.

Thus, each recorder unit periodically is activated by a correspondingmeasurement signal to position its pen in accordance with the signallevel. The position feedback signal is derived through a conductivefeedback circuit, as in FIG. 5 herein, so that the pen will always bedriven to a position corresponding directly to the applied measurementsignal. During times intervening such activation of the recorder units,the associated memory devices 30 hold the pens securely in positionpreviously set, so as to continue proper read-out and recording of themeasurement data. During those times when any recorder pen is beingshifted to a new position, the friction characteristics of the magneticmaterial provides superior servo response, as described in more detailhereinabove, to assure accurate positioning within the shortened actiontime available in a time-sharing system.

It will be evident from the above description of several embodiments ofthe invention that an important element of the invention is the use ofsmall magnetizable particles to provide friction effects creating aunique memory for signal levels representing the magnitudes of physicalconditions and the like. The availability of small magentizableparticles has of course been well known for a long time prior to thepresent invention. For example, U.S. Pat. No. 2,500,953 shows magneticparticles used in a so-called magneto-resistor. U.S. Pat. No. 2,575,360shows magnetic particles used in a torque and force transmitting device(i.e. a magnetic clutch). U.S. Pat. No. 2,667,237 shows magneticparticles used in a magnetic fluid shock absorber and related devices.U.S. Pat. No. 2,629,552 shows a magnetic fluid clutch employed in acontrol system. U.S. Pat. No. 2,603,103 shows a variable inertia deviceusing magnetizable particles. U.S. Pat. No. 2,792,565 shows fluidizedmagnetic particles solidified by magnetism to prevent any movement of amechanism; this patent also shows a variable resistance unit includingmagnetizable carbonyl iron suspended in oil. U.S. Pat. No. 2,996,267shows a vibration dampening mechanism including a variable-viscositydashpot having fluidized magnetic particles. None of these patentsdiscloses the present invention.

Although several specific embodiments of the present invention have beendescribed hereinabove in considerable detail, it is desired to emphasizethat such descriptive material has been presented for the purpose ofteaching the concepts and advantages of the invention, and should not betreated as necessarily limiting of the invention since it is clear thatmany variants and modifications of the invention can be devised by thoseskilled in the art to meet the requirements of quite differentapplications.

I claim:
 1. Signal-processing apparatus for developing, storing inmemory, and presenting a read-out of signals representing quantitativeinformation, such as the magnitude of a physical condition or the like,said apparatus comprising, in combination:signal-producing meansresponsive to a changeable condition to develop a control signal havinga variable characteristic the magnitude of which is related to themagnitude of said condition; an actuator operable by said control signalfor producing a positioning force; a movable member coupled to theoutput of said actuator such that the position of said movable member isaltered in response to changes in said characteristic of said controlsignal; magnetically-operated friction means coupled to said movablemember and including means to develop to friction restraining forcehaving a magnitude sufficient to hold said member firmly in place in theabsence of said positioning force applied to said member by saidactuator but insufficient to prevent movement of said member in responseto application of said positioning force, said friction means includinga mass of magnetized particles engaging an element coupled to saidmovable member for continously corresponding movement therewith todevelop said friction force by surface contact between said element andthe particles, said particles being supported to provide for movementrelative to one another and also to accommodate movement of said elementrelative to said particles as said element moves with said movablemember, said particles and said movable member cooperatively providing anon-drift memory with the friction-held position of said memberrepresenting the magnitude of said characteristic of the control signaldeveloped by said signal-producing means; and read-out means associatedwith said movable member for producing an output signal having acharacteristic the magnitude of which corresponds to the position ofsaid movable member.
 2. Apparatus as in claim 1, wherein said frictionmeans comprises magnetic formed into two pole pieces defining an air-gapwithin which said magnetic particles are located;said element comprisinga relatively thin elongate means mounted for movement within saidair-gap, parallel to the pole pieces thereof; and magnetic means forproducing magnetic flux through said magnetic material and across saidair-gap.
 3. Apparatus as in claim 2, wherein said magnetic meansdevelops a flux of constant intensity unaffected by movements of saidelement within said air-gap.
 4. Apparatus as in claim 3, wherein saidmagnetic means comprises a permanent magnet associated with saidmagnetic material.
 5. Apparatus as in claim 1, wherein said magneticparticles are at least approximately spherical.
 6. Apparatus as in claim5, wherein the diameters of said particles are in the range of about0.001.
 7. Apparatus as in claim 1, wherein said particles are dry. 8.Apparatus as in claim 1, wherein said particles are iron oxide. 9.Apparatus as in claim 1, wherein said element is non-magnetic. 10.Apparatus as in claim 1, wherein said particles are magnetizedtransversely with respect to the direction of movement of said element,said particles being arranged by the magnetic field into string-likechains extending transversely away from said element and permittingslight movements of said element, with a stretching effect on saidchains, without developing maximum friction force.
 11. Apparatus as inclaim 1, wherein said read-out means comprises pointer means mounted formovement with said member.
 12. Apparatus as in claim 11, including anadditional read-out means comprising sensing means responsive to thepositioning of said movable member for producing anenergy-representational signal corresponding to the position of saidmember.
 13. Apparatus as in claim 12, including manually-operable meansfor adjusting by hand the position of said movable member.
 14. Apparatusas in claim 1, wherein said read-out means comprises a sensing deviceresponsive to the positioning of said movable member and operable toproduce an energy-representational signal corresponding to thepositioning of said member.
 15. Apparatus as in claim 1, wherein saidactuator is a motor controllable by an electrical signal to apply tosaid movable member a force in either of two opposite directions. 16.Apparatus as in claim 15, wherein said motor produces a force ofconstant magnitude in either direction.
 17. Apparatus as in claim 1,wherein said movable member is an elongate member mounted for pivotalmovement in response to forces from said actuator. continuously frictionprovide
 18. Apparatus as in claim 17, wherein said element in said massof particles is a thin vane-like component of non-magnetic materialsecured to said pivotally-mounted member.
 19. Apparatus as in claim 17,wherein said read-out means comprises a pointer secured to saidpivotally-mounted member; anda scale mounted adjacent said pointer toprovide visual information respecting the magnitude of the storedsignal.
 20. The method of signal processing wherein signal informationof a quantitative nature is stored in a memory for aiding the processingfunction and wherein the stored signal information is to be read out toindicate the magnitude of a physical condition or the like;said methodcomprising the steps of; developing in response to a changeablecondition a control signal having a variable characteristic themagnitude of which is related to the magnitude of said condition;developing a positioning force in response to changes in said variablecharacteristic of said control signal; applying said positioning forceto a movable member to produce movement thereof reflecting changes insaid variable characteristic; continuously resisting changes inpositioning of said movable member by a friction force developed by amass of magnetized particles which are movable relative to one anotherand physically engage an element which is movable relative to saidparticles and is coupled to said movable member for continuouslycorresponding movement therewith, thereby to produce by mechanicalcontact between the particles and the element surface a frictionrestraining force which is applied through said element to said movablemember, said friction restraining force being (1) smaller than saidpositioning force, and (2) sufficiently large to hold said movablemember firmly in position in the absence of said positioning force, saidfriction restraining force providing a non-drift memory with thefriction-held position of said member continuously reflecting changes inthe magnitude of said characteristic of said control signal; andproducing an output signal having a characteristic the magnitude ofwhich corresponds to the position of said movable member while held inposition by said friction restraining force.
 21. The method of claim 20,wherein the friction restraining force is developed by a mass ofmagnetized particles disposed in generally-parallel string-like chainsextending away from said element transversely with respect to thedirection of movement thereof to permit slight movements of said elementwith a stretching effect on said string-like chains so as to produce forsuch sligth movements only a moderate restraining force, less than themaximum friction force.
 22. The method of claim 21, wherein saidparticles are arranged in said string-like chains by a magnetic fieldthe lines of force of which extend in said transverse direction. 23.Signal-responsive apparatus with magnetic friction memory comprising, incombination:signal-producing means to develop a control signal; anactuator operable by said control signal for producing a positioningforce either in one direction or the reverse direction; a member movablein said one direction or the reverse direction by the positioning forceof said actuator; magnetically-operated friction means coupled to saidmovable member and including means to develop a friction restrainingforce having a magnitude sufficient to hold said member firmly in placein the absence of said positioning force applied to said member by saidactuator but insufficient to prevent movement of said member in responseto application of said positioning force, said friction means includinga mass of magnetized particles engaging an element coupled to saidmovable member for continously corresponding movement therewith todevelop said frictiion force by surface contact between said element andthe particles, said particles being supported to provice for movementrelative to one another and also to accommodate movement of said elementrelative to said particles as said element moves with said movablemember, said particles and said movable member cooperatively providing anon-drift memory serving to accommodate continuous movement of saidmovable member as long as a control signal is applied to said actuatorto produce a force against said member and further serving when saidforce-producing signal ends to hold said movable member in the positionwhich it has reached at the time the signal ends, regardless of thedirection the movable member is caused to move by the signal-producedforce; and read-out means associated with said movable member forproducing an output signal having a characteristic the magnitude ofwhich corresponds to the position of said movable member. 24.Signal-responsive apparatus with magnetic friction memory comprising:amotor operable by an input signal for producing a positioning force inforward or reverse direction responsive to a signal; a member to bemoved through a range of positions in forward or reverse direction inresponse to said positioning force; means mounting said member for suchmovement without effective restraint such that said member will continueto move towards one end or other of said range of positions as long assaid motor produces said positioning force; magnetically-operatedfriction means coupled to said movable member including means to developa friction restraining force for either direction of movement of saidmember with a magnitude sufficient to hold said member firmly in placein the absence of said positioning force applied to said member by saidmotor but insufficient to prevent movement of said member in response toapplication of said positioning force, said friction means including amass of magnetized particles continuously engaging an element coupled tosaid movable member for continuously corresponding movement therewith todevelop said friction force by surface contact between said element andthe particles, said particles being supported to provide for movementrelative to one another and also to accommodate movement of said elementrelative to said particles as said element moves with said movablemember, said particles and said movable member cooperatively providing anon-drift memory through the friction-held position of said member; andread-out means associated with said movable member for producing anoutput signal having a characteristic the magnitude of which correspondsto the position of said member.
 25. Signal-handling apparatuscomprising, in combination:a movable member mounted for movement in onedirection or the reverse in response to a positioning force appliedthereto in said one direction or the reverse; magnetically-operatedfriction means coupled to said movable member and including means todevelop a friction restraining force having a magnitude sufficient tohold said member firmly in place in the absence of the positioning forceapplied to said member but insufficient to prevent movement of saidmember in response to application of the positioning force, saidfriction means including a mass of magnetized particles engaging anelement coupled to said movable member for continuously correspondingmovement therewith to develop said friction force by said contactbetween said element and the particles, said particles being supportedto provide for movement relative to one another and also to accommodatemovement of said element relative to said particles as said elementmoves with said member, said particles and said movable membercooperatively providing a non-drift memory to hold and maintain theposition of said member wherever it may be when said positioning forceceases, whichever direction the member had previously been moved by saidforce; a transducer effectively coupled to said movable member andresponsive to the position thereof for producing an output signal havinga characteristic the magnitude of which represents the position of saidmovable member; and means to transmit said output signal to a remotelocation.
 26. Signal-handling apparatus comprising, in combination:anactuator for producing a positioning force in one direction or thereverse direction in accordance with a control signal; a member movableby the force produced by said actuator, in either direction of saidforce; magnetically-operated friction means coupled to said movablemember and continuously operable to develop a friction restraining forcehaving a magnitude sufficient to hold said member firmly in place in theabsence of force applied to said member by said actuator butinsufficient to prevent movement of said member in response to theapplication of such actuator force, said friction means including a massof magnetized particles engaging an element coupled to said movablemember for continuously corresponding movement therewith to develop saidfriction force by surface contact between said element and the particlesfor movement of said member in either said one direction or the reverse,said particles being supported to provide for movement relative to oneanother and also to accommodate movement of said element relative tosaid particles as said element moves with said movable member, saidparticles and said movable member cooperatively providing a non-driftmemory with the friction-held position of said member being responsiveto said control signal developed by said signal-producing means; meansfor sensing the position of said member and for producing a feedbacksignal corresponding to the sensed position; and means for comparingsaid position feedback signal with an input signal reflecting anintended position for said member; said comparison means including meansoperable to produce said control signal for said actuator indicatingwhether the position of said member deviates from said intended positionand, if so, the direction of the deviation, said actuator responding tosaid control signal to produce said positioning force in a direction tomove said member from its sensed position to said intended position atwhich position said control signal goes to a value causing said actuatorforce to cease and said member thereafter is held firmly in saidintended position by said magnetically-operated friction means.